1. Field of the Invention
The present invention relates to a semiconductor device using a sapphire substrate and a method of manufacturing the same and, more particularly, a semiconductor device such as a light emitting diode or an integrated circuit using a sapphire substrate and a method of manufacturing the same.
2. Description of the Prior Art
In recent years, a gallium nitride (GaN)-based III-V compound semiconductor such as GaN, In.sub.x Ga.sub.1-x N, or Ga.sub.x Al.sub.1-x N attracts attention as material for a blue light-emitting diode (LED) or a blue semiconductor laser diode (LD). With the use of this compound semiconductor, it has been possible to achieve the blue-emitting device having luminous intensity to some extent. The blue-emitting devices employing the GaN-based compound semiconductor have used the sapphire (Al.sub.2 O.sub.3) substrate as a substrate. Several proposals have been set forth in Patent Application Publication (KOKAI) 4-321280 and others. A basic structure of the conventional LED is shown in FIG. 1. More particularly, a blue light-emitting device 2 consists of an n-type GaN semiconductor layer 202 and a p-type GaN semiconductor layer 203, both being laminated on a sapphire substrate 100 via a buffer layer 201. A blue light can be emitted by injecting carriers into a pn-junction region formed between the n-type GaN semiconductor layer 202 and the p-type GaN semiconductor layer 203.
In order to manufacture such blue light-emitting device, first the preselected sapphire substrate 100 is prepared, then respective gallium nitride semiconductor layers 201, 202, 203 are laminated on the sapphire substrate 100 by metalorganic chemical vapor deposition (MO-CVD) method and the like, then the laminated substrate is taken out from a reaction chamber of the CVD apparatus, then the resultant laminated substrate is cut out to be separated into individual chips of an appropriate size, and finally these chips are then connected to wire frames and are subjected to necessary wirings, molding, etc. Thus the blue light-emitting device has been finished as the product.
In the meanwhile, remarkable progress in high integration density has been achieved in recent semiconductor integrated circuit technology, especially dynamic random access memory (DRAM) technology which begins to enter into a gigabit integration. However, with the progress in integration degree, a memory cell-area of the DRAM is prone to be reduced more and more. Therefore, it is difficult to assure memory cell capacity for compensating for disappear of memory contents caused by alpha (.alpha.)-particles which exist in the natural world, i.e., so-called soft error. Hence, as shown in FIG. 2, a semiconductor device is often fabricated on a single crystal silicon film formed on the sapphire substrate because it is feasible to match lattice constant of silicon crystal with lattice constant of the sapphire. Since a so-called SOS (Silicon-On-Sapphire) device shown in FIG. 2 can be miniaturized and operate at high speed, it would be promising as a high performance device. Because of its structure, the SOS device must be formed to use the single crystal Si layer formed on the sapphire substrate as the active region. Therefore, such several advantageous merits will be expected that devices like transistors formed in the active region may be perfectly isolated, stray capacitance between the substrate and the circuit can be lessened if the integrated circuit, etc. are formed thereon, latch-up can be suppressed in CMOS, and the like. In addition, since the SOS device may restrict electron-hole pairs generated by .alpha.-particles within a thin single crystal silicon film which is formed on the sapphire substrate, .alpha.-particle immunity or the soft-error-rate reduction in DRAM, and so on can be improved remarkably. FIG. 2 is a sectional view showing the DRAM cell having the SOS structure. A silicon film 303 is epitaxially grown on a sapphire substrate 100. An n.sup.+ type source region 306 and n.sup.+ type drain region 316 are then formed in the silicon film 303. A data line (bit line) 409 is then formed on the n.sup.+ type drain region 316 via a contact electrode 408. A storage electrode 405, a capacitor insulating film 406, and an opposing electrode (plate electrode) 407 are formed on the n.sup.+ type source region 306 via a contact electrode 410. A gate electrode 305 made of polysilicon, etc. is formed on a channel region between the n.sup.+ type source region 306 and the n.sup.+ type drain region 316 and on the silicon film 303 via a gate oxide film 304. The gate electrode 305 also serves as a word line of the DRAM.
In the conventional semiconductor device such as LED or DRAM using the sapphire substrate as described above, it is considerably hard, due to hardness of the sapphire substrate, to cut off the sapphire substrate on which semiconductor layers are laminated. Usually the sapphire substrate is cut off by the diamond cutter. Nevertheless, the sapphire substrate must be polished very thin in advance of cutting, up to less than 250 .mu.m (even though it may be thick), for example, or up to about 100 .mu.m as the case may be. But such problems have arisen that it is very difficult in respects of mechanical strength, etc. to polish the sapphire substrate up to such thin thickness and, therefore, not only is it probable that the growth layers acting as the light-emitting layer and the channel region are distorted or strained with such polishing, but also considerable manufacturing time is required. Accordingly, the cost of production has gone up, which is one of bars to mass production of the semiconductor device formed on a sapphire substrate.